Scientists have long suspected that genetic and environmental factors work together to cause rheumatoid arthritis (RA). While the exact mechanisms remain a mystery, evidence in support of gene-environment interactions now suggests there may be many pathways to RA. One risk factor strongly implicated in causing RA is smoking. A large body of research persuasively shows the role cigarette smoking can play in RA if the genetic climate is right. Now, scientists are trying to understand how smoking might trigger a particularly aggressive form of RA in certain people.

Gene-environment Interaction in RA

The search for genetic risk factors for RA began in the HLA (human leukocyte antigen) system. HLA molecules play a key role in immune function, and variations in HLA genes have been found in many autoimmune and inflammatory diseases. Sure enough, the first gene linked with RA – the so-called shared epitope (SE) gene – was found in the HLA system. More than 30 genetic risk factors for RA have been revealed since, but the SE gene remains the one most strongly associated with the disease.

But scientists know it takes more than genes to cause RA, as studies in identical twins show. Despite having the same set of genes, an identical twin of someone with RA has roughly a 15 percent risk of also developing RA, indicating that other factors must be at play.

“Even the strongest genetic risk factors are not seen in all people with RA,” explains Ted Mikuls, MD, a rheumatologist and epidemiologist at the University of Nebraska Medical Center in Omaha. “This is true of every risk factor for RA that I am aware of,” he adds. “That tells you just how complex this disease is. There isn’t a single risk factor that explains it.”

Dr. Mikuls is one of a handful of worldwide researchers studying how genetic factors and environmental exposures may influence the risk of RA. Over the past 20 years, several large population-based studies of gene-environment interactions in RA have been conducted in the United States and throughout Europe. Increasingly, these studies show that RA is caused by an interaction of many factors.

Of all environmental factors suspected or described, smoking is by far most convincingly linked with RA. Study after study has implicated smoking as a contributor to the disease in certain people.

“The case for smoking being a risk factor for developing RA is pretty clear cut. This has been a fairly consistent and robust finding,” says Dr. Mikuls. He notes that the evidence is strengthened by the number and quality of the studies linking smoking and RA and by the fact that the studies have been done in varied populations – men and women, Caucasians and African-Americans, and people from many countries.

“But, it’s not just smoking,” adds Dr. Mikuls. “It’s smoking in the context of the right genetic factors – and probably many factors we’ve yet to identify that mediate this risk.”

One factor that often coexists with smoking is the SE gene. Dr. Mikuls describes the smoking/SE gene duo as being “a particularly evil combination in terms of disease risk” because it frequently shows up in people with the more aggressive form of RA that involves formation of antibodies to citrullinated protein antigens (ACPAs). The risk for ACPA-associated RA increases with the total amount of smoking exposure and the number of SE gene copies (zero, one, or two). Smokers with two copies of the SE gene (one from each parent) have 21 times the risk of ACPA-associated RA as nonsmokers with no copies of the SE gene.

Smoking as a Trigger

No one knows for sure how genes and smoking work together to ignite the spark of RA, but many researchers speculate about how smoke exposure might trigger faulty immune functioning and autoantibody production in a genetically predisposed person. One leading theory involves smoking, the SE gene, and “citrullinated” proteins.

A citrullinated protein is a protein that has undergone a change in one of its amino acids (a switch from arginine to citrulline). This change puts a tag on the protein, letting it be spotted by immune system scouts looking to get rid of old, defective, or problematic cells. Although citrullination is a normal cellular process, several inflammatory diseases are associated with increased levels of citrullinated proteins.

There is growing interest in examining the role citrullinated proteins play in RA. Antibodies to these proteins may appear in the blood many years before symptoms of RA emerge. In addition, studies have shown that:

- Citrullination makes certain proteins more prone to bind to SE gene–containing molecules

- Citrullination of a collagen protein causes the protein to be capable of triggering arthritis in mice

- Citrullinated proteins are found on many cells collected from the airways (lungs) of smokers but not on cells from the airways of nonsmokers

A group of Swedish researchers put this information together to propose a theory of how smoking might trigger RA in a person with a genetically altered immune system. According to this theory, prolonged exposure to cigarette smoke causes formation of a large number of citrullinated proteins that, in the presence of altered immunity, trigger an autoimmune response (antibody formation) that ultimately targets the joints and other tissues for autoimmune attack.

This theory has led researchers to propose further theories, one of which is that initial steps in RA development may occur in the lungs, and that smoking may be part of a trigger mechanism. A research team at University of Colorado in Denver is investigating this theory.

Kristen Demoruelle, MD, a second-year rheumatology fellow on the Denver research team, describes the group’s work as “preclinical” because it involves studying people who have not developed symptoms of RA but are considered at high risk for the disease because they have a family member with RA and test positive for ACPAs or for two or more versions of the rheumatoid factor antibody. In previous studies by the group, the presence of these high-risk autoantibodies predicted the future development of RA. The team is trying to understand how and why these autoantibodies appear before RA starts.

“We know that autoantibodies are present in most people who develop autoantibody-positive RA months or even years before they develop any sign of joint disease. “Something had to happen prior to this,” says Dr. Demoruelle.

“Some event, which could be an environmental exposure like smoking or an infection, in a person with the appropriate genetic factors may lead to the development of autoimmunity and, eventually, the symptoms of RA,” she explains.

The team has found that people without RA who have the high-risk autoantibodies are much more likely to have evidence of inflammation in their lungs than people without RA who do not have the high-risk autoantibodies. These findings suggest that there is something happening in the lungs in the preclinical period, and the group is trying to figure out what it is and what it means.

“Our goal is to look closer at the mechanisms of RA development to see if there is something occurring in the lung that could be involved in starting the disease,” explains Dr. Demoruelle.

Dr. Mikuls acknowledges that researchers still have many questions about the mechanistic links between cigarette smoke and development of RA.

“We know they are associated, but we don’t know all the steps or how they fit together,” says Dr. Mikuls. “Many hypotheses are being explored, such as involvement in the lung, increased citrullination of proteins caused by cigarette smoking and stronger autoantibody responses to these proteins in people who carry the HLA genetic risk factors. And those are just some steps we think we know. There are a lot of fine details between those steps we haven’t gotten to.”

Could Avoiding Smoke Exposure Prevent RA?

A big take-home message from the many large gene-smoking studies, particularly a large study in Sweden, is that smoking is a major preventable risk factor for RA. The Swedish research suggests that 20 percent of cases of RA overall and 35 percent of cases of ACPA-associated RA are due directly to smoking. When the researchers looked just at ACPA-associated RA, they estimated that 55 percent of cases involving two copies of the SE gene are due directly to smoking.

“That’s pretty amazing if you think about it,” says Dr. Mikuls. “If you could abolish smoking as an environmental hazard, you could get rid of one in five new cases of RA. If you went to the population of people with two copies of the shared epitope gene and could abolish smoking, you would eliminate one in two new cases of RA. That’s amazing.”

Whether avoiding second-hand smoke also could help prevent RA in genetically predisposed people is unknown.

“We don’t know if second-hand smoke is a risk factor for RA, but this is a great question,” says Dr. Demoruelle. “Unfortunately, our ability to study this question is limited by the fact that it is extremely difficult to classify how much second-hand smoke a person has been exposed to.”